Vehicle immobilisation devices of the type defined are disclosed in U.S. patent application No. 3,695,071 and GB-A-No.2,112,725. In U.S. patent application No. 3,695,071, the relatative positions of the chocks and top hook can be adjusted to suit the diameter of the particular vehicle wheel to which the device is to be applied. The arrangement involves three arms connected to the chocks and top hook, respectively. Two arms have sets of spaced slots, and the third arm has a tenon which is adjustably mounted thereon. To obtain the necessary spacing between the parts, all three arms and the chocks and top hook have to be placed in position and the tenon adjusted and passed through the correct slots in the other two arms, before locking can take place. Thus the device cannot readily be presented to a vehicle wheel of unknown size, adjusted quickly and locked thereto.
GB-A-No.2,112,725 describes an arrangement in which two chocks are joined together by telescopic members which can be locked together by aligning holes therein and inserting the straight portion of a stem or locking bar carrying the top hook through the aligned holes. The problem of aligning two holes in telescopic members sufficiently accurately to receive a well fitting bar is clear. The device actually manufactured by the applicants of the application GB-A-No.2,112,725 used a locking arrangement differing slightly from that described in the application. A pin on the locking bar still passed through a pair of alignable apertures (a plurality being provided) in the telescopically arranged chock arms, but a separate plate was provided depending from the outer of the telescopic arms. The shield extended down to the plate, an aperture in the shield aligned with one of a plurality of apertures in the plate and a padlock was passed through the aligned shield and plate apertures.
An alternative arrangement is described in application GB-A-No.2,112,725 whereby respective chock arms on each chock are connected in a telescopic manner by a separate tube. When the device is fitted the two arms lie side by side having entered the tube from opposite sides thereof, and are slidable in the tube to adjust the distance between the chocks. The top of the tube has a slot in it and the two arms are notched on their upper surfaces in at least one position each. When the device is correctly fitted, notches in the arms line up with the slot in the tube, and means is provided to engage the slot and notches to lock the arms against the separation.
Both these arrangements depend for their locking on the chock arms being in the correct position relative to the locking means for locking to be possible.